More than 86% of electricity in the U.S. is produced in thermoelectric power generating plants, most of which use coal, natural gas, or nuclear power to generate thermal energy. The thermal energy drives steam turbines to produce electrical power, and typically, more than 60% of the original energy is wasted and dissipated as low-grade heat. Operators must remove this heat, and 99% of baseload thermoelectric plants in the U.S. use water-cooled systems, or wet cooling, to do so. As a result, wet-cooling systems at power plants currently account for 41% of all freshwater withdrawals in the U.S. Since availability of freshwater resources is increasingly strained by drought and growing demand, and potential climate change impacts add uncertainty to the quality and quantity of future water supplies, dry-cooling systems (air-cooled condensing) have therefore received increased attention. In these systems, however, the air temperature tends to be warmer than the water temperature, and air has a substantially smaller cooling capacity compared with that of water. As a result, current dry-cooling technologies are less effective in cooling the steam in power plants, thus leading to a reduction in the efficiency of power generation compared with that for water-cooled generators. Specifically, during hot ambient temperatures, the efficiency of an air-cooled condenser is lower, because the temperature difference between the air and the steam is low. As a result, power plants are overdesigned with respect to air-cooled condenser cooling capacity, so they can handle the highest ambient temperatures. Unfortunately, this dramatically increases the capital cost of dry cooling systems.
Thermal energy storage systems store and release thermal energy by heating or cooling the thermal storage medium so that the stored energy may be used at a later time for heating, cooling, or power generation applications. By storage mechanism, the storage technology may be classified into three groups: sensible heat storage systems, latent heat storage systems, and thermochemical storage systems.
Sensible heat storage systems store and release thermal energy by a temperature increase or decrease of the storage medium. The storage medium may be a liquid, a solid, or a gas. Conventional sensible heat storage materials include water, molten salt, sand, rock, concrete, and metals.
Latent heat storage systems store and release thermal energy in the latent heat of the storage material, which undergoes a phase change. The phase change may be a solid-liquid phase change or a liquid-gas phase change. Conventional phase change materials (PCMs) include hydrated salts, paraffin waxes, fatty acids, and eutectics of organic and non-organic compounds.
Thermochemical storage systems store and release thermal energy by reversible exothermic and endothermic reactions. The main principle of thermochemical energy storage is based on the chemical reaction: C+heat↔A+B. In this reaction, the thermochemical storage medium C absorbs heat and is converted chemically into components A and B. The reverse reaction occurs when materials A and B react to form C with the release of heat. Several types of reactions have been investigated for storage of thermal energy, including dehydration, metal hydroxides, and metal oxides. Conventional thermochemical storage materials include magnesium sulfate heptahydrate (MgSO4.7H2O), ferrous carbonate (FeCO3), calcium hydroxide (Ca(OH)2), and manganese dioxide (MnO2).
Phase change materials have received substantial attention for thermal energy storage, since such materials are readily available, may be inexpensive to obtain, and have a relatively high heat storage capacity. When choosing phase change material candidates, the importance of various criteria may vary on a case-by-case basis. General guidelines to follow may include having a thermal storage system with desirable heat transfer performance that is stable through repeated thermal cycling and available at a reasonable cost. Significant research and development has been focused on using paraffin materials as well as salt hydrates. Salt hydrates are a group of inorganic materials that are inexpensive, have a relatively high thermal conductivity, and are environmentally friendly.